1. Field of the Invention
The present invention relates to a method for determining optical turbulence, as measured by the refractive-index structure parameter, of the atmosphere along a beam propagation path using a simple laser and corner cube system and more particularly such a system involving a particularly sized receiver and corner cube combination.
2. Related Art
A laser beam propagating through the atmosphere encounters random refractive-index fluctuations. These random fluctuations are often referred to as optical or atmospheric turbulence. Such turbulence in the surface layer of the atmosphere, extending upward from the ground to a few tens of meters, is unstable if heat is rising from the ground, as is common during sunshine where the rising heat adds to the turbulence. The surface layer is more stable if the ground is cooler than the air, so that heat is transferred downward, as is common at night and such stability tends to damp turbulence. The surface layer is near neutral if wind shear alone produces almost all of the turbulence, whereas the heat transfer between ground and air is not effective in producing or damping turbulence—which condition typically occurs when the wind is strong and/or solar radiation is weak.
Optical or atmospheric turbulence cause scintillation or twinkling of distant light sources. The strength of this turbulence that produces scintillation is represented by the refractive-index structure parameter, which is denoted by Cn2. In particular, this parameter, Cn2 reflects the optical turbulence impacting a laser beam's characteristics as it is transmitted through a particular region. As a result, measurement of atmospheric parameters to describe the optical turbulence to which a laser beam is subjected as it propagates during field experiments is critical for understanding and predicting the atmospheric effects on the optical wave.
A variety of in situ techniques are known for measuring atmospheric fluxes, including using a sonic anemometer or performing indirect-dissipation measurements using a fine wire thermometer and hot-film anemometers. These prior art methods, however, require excessive capture, processing, and analysis of high data rates. Also, the instruments are not particularly rugged. For example, fine-wire thermometers are subject to hygroscopic-particle contamination. And, finally, such in situ measurement of fluxes at a single point yields point measurements of fluxes which are extremely location dependent. Movement from one location to another produces different readings. To obtain more general readings, for example as part of an ecological or air quality study, it is required to take measurements at a large number of points and to average the resultant data, making the overall collection of data rather cumbersome.
To overcome such prior art problems, U.S. Pat. No. 5,150,171 discloses a method of deriving the refractive-index structure parameter using a scintillometer, which includes a laser light source and a detector aligned to receive light from the laser light source through the atmosphere. A source of incoherent light is aligned with a second detector. One detector circuit measures fluctuations in intensity of the light received from the laser light source; while, a second detector circuit measures fluctuations in intensity of the received incoherent light. The inner scale of turbulence is obtained from a ratio of the two measured variances. With the known inner scale of turbulence, the system utilizes a processor to derive the refractive-index structure parameter from the measured variances of intensity.
While the scintillometer of U.S. Pat. No. 5,150,171 does overcome some of the problems of the prior art, it is complex and necessitates the use of expensive equipment; such that there is a need in the art for a simpler and more cost effective means of measuring Cn2.